The E. coli bacteria gets a bit of a bad rap, since you only ever hear about it when you're sick.
In reality, there are many different kinds of E. coli that live in your intestines all the time, synthesizing fatty acids to aid digestion and producing vitamins; E. coli has even been suggested as a starting point for producing biodiesel fuel.
But they're not all helpful: some forms of E. coli can cause bloody diarrhea, kidney failure, or urinary tract infections. Like salmonella, these forms of E. coli are transmitted through the feces of animals found on food or in undercooked meat.
The cytoplasm of the gram-negative bacterium Escherichia coli. Photo: Adrian H Elcock (University of Iowa) via Flickr